Multi-layer laminates such as carbon fiber-reinforced composites (CFRP) and fiberglass composites (FRP) are widely used in a large number of applications. These laminate materials most often consist of woven layers of strong fibers that are often coated with resins and processed or cured to form a solid structure. Depending on the choice of the fiber and the resin systems used, these materials can be formulated and molded to produce components with excellent mechanical properties and unique geometries that would be difficult or impossible to obtain using other materials.
The properties of high strength CFRP materials may be widely varied by manipulating the characteristics of the matrix formulation, as well as the fiber type, content, orientation, buildup, and the methods used to shape these materials into a finished structure. This variability and the general strength of the CFRP materials make them useful in a wide variety of applications, ranging from bicycle frames to aircraft structures.
The reinforcing fiber most widely used in aircraft structures is a carbon fiber produced by the thermal decomposition of polyacrylonitrile (PAN). Such thermal decomposition coverts the PAN fiber to a pure carbon fiber that is highly abrasive and very strong. In some specific examples, such carbon fibers are reported to have tensile strengths of about 800,000 psi and a modulus of about 40 million psi. Such carbon fiber materials are produced by a number of companies such as Toray, Toho Tenax, Cytec, Hexcel, and Mitsubishi Rayon.
In producing structures such as aircraft components, these high-strength fibers typically are first woven into thin sheets and combined with resins to form flat sheets of composite referred to as “prepregs”. Components such as composite skin sections of aircraft may be produced by placing multiple layers of such prepregs in molds and then using pressure and heat to shape and cure them into a complex wing surface, for example. Alternatively, components may be constructed by chopping carbon fibers into shorter lengths and blending them with resins to produce a compound suitable for use in compression molding or resin-transfer molding.
CFRP laminate parts have been used in the manufacture of aircraft for several years. In one example, the 777 aircraft manufactured by Boeing uses CFRP for the passenger cabin floor beams, for the vertical and horizontal tails, and for aerodynamic fairings. Overall, CFRP-based components make up about 9% of the structural weight of this aircraft.
Composite components such as aircraft parts are often joined together or to other materials by fasteners. Processes used to join such components generally include the steps of drilling and countersinking a precision hole in the structures to be joined and then inserting a close-fitting fastener in a secure manner. Drilling of CFRP components is often difficult as a result of the highly abrasive nature of the material and has a tendency to delaminate and fray when processed using conventional drill bits. One of the more serious problems experienced in drilling CFRP occurs when the exit of the drill bit from the produced hole leaves uncut fibers exposed in the hole. Such fibers then may interfere with the proper fit of the fastener used to join the materials.
While many of the components lend themselves to being manufactured with NC or CNC drilling machines, there remains a portion of the holes in the structure that cannot be manufactured with such equipment and may require a hand held air drill motor to be used. Such drill motors are produced by companies such as Cooper Tools and are often used in conjunction with a hand held guide bushing.
When drilling holes with a hand drill in CFRP, the infeed of the drill bit into the material may be regulated by the operator who forces the drill bit into the material. Unfortunately, the drill bit may often surge at the point it exits the material on the backside, due to a lack of a controlled feed, resulting in uncut fibers. Even with considerable skill and experience, an unacceptable hole is often produced by this method.
The existing practice is to use a four flute straight flute drill bit design. (See FIG. 1). Such straight flute drill bits are often difficult to control at exit and in addition may produce a star shape hole as opposed to a round hole in the material. The uncut exit fibers combined with the star shaped hole may result in a less than optimum fit of the fastener in joining the materials.
Hole quality often has a direct bearing on the fatigue properties of the fastened joint. Such properties are documented by fatigue tests. Fatigue results for parts fastened with inferior hole quality. Such inferior hole quality often shows a marked reduction in fatigue life adversely affecting the suitability of the components in advanced aircraft structures.
As a result, it would be desirable to provide drill bits and methods of their use to produce cleaner holes with an improved roundness. Furthermore, it would be desirable to provide drill bits and methods of their use to produce holes using hand drill motors in advanced composite materials such as CFRP that improve the roundness of the hole and eliminate uncut fibers at the exit allowing for an improved fit between the fastener and the hole, thereby resulting in improved fatigue results for these joints.